The invention relates to a vehicle electrical system for supplying electrically operated loads in motor vehicles.
A known vehicle electrical system of this type (DE 196 51 612 A1) comprises a common voltage supply terminal for the entire vehicle electrical system to which, preferably, a vehicle electrical system control unit is connected, said control unit then itself supplying the loads which are connected to it. The vehicle electrical system control unit can comprise additional voltage converters which convert the general power supply voltage fed to it into various voltages depending on the requirements of the loads which are to be respectively supplied. In order to ensure that the various electronic and electrical components are safely and reliably supplied with a voltage, a plurality of voltage supply sources, which may possibly also have different voltages, are provided, one of which is connected in each case to the voltage supply terminal for the vehicle electrical system control unit.
For this purpose, sensors are provided in the form of Zener diodes and other associated transistors which detect the failure of a power supply source when a switch, usually an ignition switch, closes and switch over the common voltage supply terminal to another voltage supply source.
The practice of embodying vehicle electrical systems with two or more circuits which usually have different supply voltages is generally known and efforts are being made to promote it, in particular in view of the continuously rising demand for electrical power in motor vehicles. In this respect, reference is made, in particular, to the paper xe2x80x9cEntwicklung kxc3xcnftiger Bordnetz-Architekturen . . . [Development of future architectures for vehicle electrical systems]xe2x80x9d in VDI reports, No. 14/15, 1998 (conference in Baden-Baden, Oct. 8 and 9 1998). For example, there have been extensive investigations into a dual-voltage vehicle electrical system (see FIG. 1 of the aforesaid paper) in which a first vehicle electrical system is provided with a relatively high supply voltage, for example 42 volts for high-consumption loads, continuous loads, heaters and for regulated loads (fans, seat motors), while a second, and in this context customary, 14 V vehicle electrical system is used to supply lower-power loads, in particular quiescent current and low-voltage loads such as filament lamps, electronic control systems, communications and the like.
Both vehicle electrical circuits can be supplied here with current by a common generator which operates directly on the high-consumption load and the starter area and which is assigned its own battery, in the specific case of a 42 V vehicle electrical system, a 36 V battery, while the 14 V loads are connected to the generator by means of a direct voltage converter, specifically a so-called DC-DC converter, and preferably have their own 12 V buffer battery.
Since devices which have previously been operated mechanically, hydraulically or even pneumatically in vehicles are increasingly being embodied as electrical devices, the demand for electrical power is determined by the actuation of these devices or by the provision of operating power (servo support), respectively, or else by both of these at the same time.
If the respective devices which are now operated electrically are systems which relate to safety, for example electromechanical brakes, electromechanical steering systems or generally so-called X-by-wire systems, for legal reasons alone it is necessary to provide redundancy, or an emergency operating mode, owing to general safety considerations.
The invention is therefore based on the object of ensuring in a multivoltage vehicle electrical system in motor vehicles that at least one emergency operating mode is provided in a vehicle electrical circuit, which is possibly at risk of failure, for the loads which are fed by said vehicle electrical circuit.
The invention achieves this object in the multivoltage vehicle electrical system mentioned at the beginning by means of a cross-connection which responds in the event of a failure of the vehicle electrical circuit and feeds electrical power from the respective other vehicle electrical system even to the loads which are assigned to the failed vehicle electrical system, at least to such an extent that a possibly reduced emergency operating mode is possible.
In this context, in the event of a failure of the vehicle electrical circuit which has a relatively low voltage, the problems incurred are not wider ranging because an uncomplicated reduction in the voltage from the vehicle electrical system which has a relatively high voltage can continue to ensure the supply voltage necessary for the electronic control system, for example by using customary DC-DC converters or in-phase regulators. In any case, the loads on the lower voltage level are predominantly lower-power ones which can also be supplied from the other vehicle electrical circuit for the period until the damage is remedied.
However, in the event of a failure of the vehicle electrical circuit with a relatively high voltage, in which circuit there are continuous loads, the electronic power system, servo systems which have a high power demand and actuator motors, the preferably direct cutting in of the lower-voltage vehicle electrical circuit which ensures the emergency operating mode is advantageous because immediate failure of the system is avoided and the only disadvantage is a relatively slow response, possibly also with reduced effect.
In any case, the failure of a respective electrical supply circuit in the motor vehicle is, of course, signalled to the operator by means of suitable indication, for example by means of a flashing indicator lamp or signalling tone, in order to find as soon as possible a repair possibility with a correspondingly reduced speed.
If appropriate, in this case it is also possible to use engine speed limiters which permit the motor vehicle to continue to operate only to a limited degree.
The basic idea of the invention consists in the fact that in the case of a multivoltage vehicle electrical system the respectively intact power supply of a vehicle electrical circuit is to be switched over, by a cross-connection which responds when necessary, to a defective vehicle electrical circuit, with the result that an emergency operating mode and a respectively increased level of operational reliability are obtained.